Cooling air is frequently used to cool the materials of the exhaust nozzles of gas turbine engines. Frequently, fan air is used where considerable pressure is available to direct and modulate the cooling air flow.
In engines with high fan pressure ratios the cooling air is extensively compressed and therefore has an elevated temperature. Accordingly, it is difficult to achieve adequate cooling with this higher temperature air.
It is known therefor to design the nozzle as an ejector whereby cooling air can be induced at or beyond the throat plane. Ambient air enters the ram duct and is induced from there into the nozzle, adjacent the surfaces. It can be appreciated that very little excess pressure is available with this arrangement.
When axisymmetric nozzles are used requiring pitch and yaw, appropriate flow becomes more tenuous. The external flaps which confine the ram air can skew and become difficult to seal. Ram air can then leak out, rather than passing into the nozzle flow.